The present invention relates to an image density control method for an image forming apparatus for controlling the density of an image by determining the density of a toner image formed on a photoconductive element by a reflection type photosensor and, if the density is low, supplying a toner to a developing unit.
In an electrophotographic copier, for example, a latent image representative of a pattern having a reference density is electrostatically formed on a photoconductive element outside of an image area. The latent image is developed by a toner to produce a toner pattern image. A reflection type photosensor senses the density of the toner pattern image and the density of the background of the photoconductive element preceding or succeeding the toner pattern image (in the circumferential direction of the element). Whether or not the image density is adequate is determined on the basis of a relation between the two sensed densities. If the image density is low, a toner is supplied to a developing unit to increase the toner concentration of a developer stored in the developing unit.
The output voltage of a reflection type photosensor is associated with the amount of toner deposition on the photoconductive element. However, such a photosensor has a problem left unsolved, as follows. When the photosensor is of the type outputting a high level voltage in response to a high density, the output voltage thereof becomes saturated in an area where a substantial amount of toner is deposited, e.g., in a solid image portion having a substantial area. On the other hand, when use is made of a photosensor of the type outputting a low level voltage in response to a high density, the output thereof approaches the base value in the above-mentioned area. In any case, the output voltage of the photosensor does not noticeably change despite the change in the density of the object, i.e., the sensitivity is low. Hence, the photosensor cannot satisfactorily respond to the increase and decrease in the amount of toner deposition. Should the sensitivity be increased for a high density area, it would be lowered in a low density area and substantially prevent the density from being sensed in an area where the amount of toner deposition is extremely small. Conversely, should the sensitivity be increased for a low density area, it would be substantially impossible to sense the density in a high density area such as a solid image portion. To eliminate this problem, it has been customary to select a lower potential for the toner pattern image than for the solid image portion to thereby form a halftone pattern image. The photosensor detects the change in the amount of toner deposition on such a halftone pattern image. To set up the potential for the halftone pattern image, the charge potential (absolute value) is reduced and/or the bias voltage (absolute value) to be applied to the developing roller is increased, compared to the potential for the image area. For example, Japanese Patent Laid-Open Publication No. 241571/1989 proposes a device for automatically changing the bias voltage for development.
As stated above, it is a common practice to assign a particular image forming condition to each of the image area and the non-image area or marginal area for forming the toner pattern image. However, the same image forming condition is assigned to the non-image area and the background, i.e., the background is processed under the condition which is optimal for sensing the density of the toner pattern image. It follows that the background allows a carrier included in the developer to deposit thereon or is excessively contaminated depending on the resistance of the carrier and the amount of charge deposited on the toner, preventing the background density from being accurately sensed.